The JPEG Still Picture Compression Standard

The JPEG paper asks how a photograph can become much smaller without looking obviously broken. Read it to see how each 8 × 8 block becomes a set of spatial-frequency coefficients, why coefficient quantization is the deliberately lossy step, and how zig-zag ordering plus Huffman coding pack the surviving structure without pretending that one quality setting fits every image or display.

Reading focus: Why the discrete cosine transform changes an 8 × 8 block's coordinates without itself discarding image information. How a 64-entry quantization table trades coefficient precision for more zeros, and why that trade depends on image and viewing conditions. How differential DC coding, low-to-high-frequency zig-zag order, and Huffman coding compress the quantized symbols without adding another lossy step.

Communications of the ACM 1991. Gregory K. Wallace. 30 min read, easy difficulty.